In the 3rd Generation Partnership Project (3GPP) serving as a standardization body for asynchronous cellular mobile communication, discussions are underway to cover not only existing wireless communication between base station (ENB) and user equipment (UE) but also wireless communication between UEs or devices (i.e. D2D communication) in the Long Term Evolution (LTE) system specification.
Public safety services are one of the main functions required for 3GPP D2D communication. That is, LTE-based wireless communication is to be available within and between groups of police officers, fire fighters, and government officials even in the case of emergencies (e.g. natural disasters) where normal network services are not available. In the case of an emergency, one-to-many broadcast communication covering an entire group or all groups is more efficient than one-to-one unicast communication between members in one group. In the 3GPP, there has been an agreement on the use of D2D broadcast communication in Release 12 specifications.
In D2D communication, a transmitting UE (Tx UE) may use one of the following two schemes for radio resources. First, there is a centralized resource allocation scheme, where a Tx UE may be allocated radio resources to use by a given resource allocation agent. Here, the resource allocation agent may perform a function similar to that of a base station in cellular communication, and a designated UE may perform the resource allocation function when the network fails to provide a normal service. In an ideal case, the UE undertaking resource allocation may perform radio resource scheduling for other UEs in its coverage, making it possible to conduct D2D communication without resource conflicts.
However, in the case of centralized resource allocation, it is necessary to separately define a scheme for selecting a UE to undertake resource allocation. As a UE undertaking resource allocation has to support the function of a base station, device complexity increases. It is also necessary to define a control channel for exchanging resource allocation information. When multiple UEs undertaking resource allocation are closely placed in a region, coordination therebetween is needed to prevent conflicts in resource allocation in the region. Base stations may exchange coordination information through wired links. However, it is necessary to define separate physical channels or signals for exchanging coordination information between UEs undertaking resource allocation.
Second, there is a distributed resource allocation scheme, where a Tx UE may directly select radio resources to use. In general, a Tx UE may select radio resources through Channel Sense Multiple Access/Collision Avoidance (CSMA/CA). That is, a Tx UE performs channel sensing on a radio resource region configured for D2D communication to identify whether the corresponding radio resource is used by another UE for D2D communication. If the corresponding radio resource is used by another UE, the Tx UE continues channel sensing to find an available (empty or unused) radio resource. If the corresponding radio resource is not used (available), the Tx UE may use the corresponding radio resource to send a signal. Here, the Tx UE using the radio resource is required to send a channel sensing signal to notify the radio resource being used to other UEs performing channel sensing. The channel sensing signal may have a sequence-based structure like a random access preamble or a reference signal.
In the case of distributed resource allocation, a resource collision may arise when multiple Tx UEs simultaneously send a signal on the assumption that a radio resource is available after channel sensing. As a UE does not have to undertake resource allocation like the function of a base station, device complexity does not increase. Distributed resource allocation may be operable through minimal signaling based on a channel sensing signal because UEs undertaking resource allocation and coordination therebetween are not necessarily required unlike the case of centralized resource allocation. In particular, a random backoff algorithm may be applied to alleviate the problem of resource collision between Tx UEs. That is, a Tx UE performs channel sensing. If an empty or unused radio resource is found, the Tx UE continues channel sensing for a randomly selected backoff time. If a channel sensing signal from another UE is not detected during the backoff time, the Tx UE starts transmission using the found radio resource. Otherwise, the Tx UE stops the backoff procedure.